A. Field of the Invention
The invention is a method of treating a subterranean formation penetrated by a wellbore. More specifically, it relates to improving fracture conductivity of a fracture in such a formation which contains a propping agent.
B. Description of the Prior Art
It is common practice to hydraulically fracture subterranean formations containing valuable fluids such as hydrocarbons. In a typical fracturing treatment, a particulate propping agent, e.g. sand, glass beads, sintered bauxite, nut shells, zirconia, alumina, and the like, is placed in the fracture to keep the fracture at least partially propped open. The proppant pack is frequently less permeable than it might be due to the presence of siliceous fines and clay platelets dispersed in the pack, resulting in less than optimum fracture conductivity. Such fines and clays may be initially present in the proppant when the proppant is placed in the fracture, or may migrate into the proppant pack from the formation as fluid is produced from the formation. Regardless of the source of the fines, and regardless of whether the proppant pack is a newly placed pack or one which has been in service for a period of time, it is desirable to increase the fracture conductivity by cleaning the proppant pack. A fluid used for such a treatment must be able to react with the fines and clays sufficiently to increase the permeability of the pack without attacking the principal propping agent to such an extent that the strength of the pack is materially reduced. Conventional mud acid (e.g. 12% aqueous HCl and 3% aqueous HF) has been used for this purpose, but has several disadvantages. Mud acid spends rapidly and therefore cannot penetrate deeply into the extremities of the fracture. Also, it provides no clay stabilization to undissolved particles, and can cause sloughing of the fracture face in certain formations. Another fluid which has been proposed in the past for such purpose is phosphoric acid containing some ammonium bifluoride.
Fluoboric acid has been proposed for various well treatments in the past, but so far as is presently known, fluoboric acid has never been proposed for use in cleaning a proppant pack. Prior art utilizing fluoboric acid in well treatments includes Ayers, Jr., U.S. Pat. No. 2,300,393; Bond, et al., U.S. Pat. No. 2,425,415; Kingston, et al., U.S. Pat. No. 2,663,689; and the commonly assigned inventions of Ronnie L. Thomas upon which Ser. Nos. 824,753 and 824,754, both filed Aug. 15, 1977, are based.